The invention relates to gas emission spectrometers for analyzing a gas stream to detect and quantify the concentration of predetermined gaseous contaminants in a gas stream of mixed gases under continuous flow conditions. In particular, it relates to an improved electric discharge tube comprising one or more sample cells for a gas emission spectrometer, featuring a plurality of analytical sites, and a method for simultaneous analysis of a plurality of selected impurities.
An ultra high purity supply of inert gas, particularly argon, has become essential in the manufacture of large scale integrated circuits. Semiconductor manufacturers utilize commercial purifiers to remove impurities in the argon stream to less than 10 parts per billion (ppb). Some of the more important impurities removed by these purifiers include O.sub.2, H.sub.2 O, CO, H.sub.2, CO.sub.2, CH.sub.4 and N.sub.2. Continuous monitoring of the inert gas stream under continuous flow conditions to assure that the gas stream purity continues to meet its stringent specifications is mandatory.
Currently the only method for continuous monitoring of nitrogen at low concentration levels in a high purity argon gas stream is emission spectroscopy. In emission spectroscopy the gases are excited in a gaseous discharge to produce optical emission lines characteristic of each gas in the gas stream. The emission line of nitrogen is then isolated and analyzed to measure its intensity in order to quantify its concentration.
The conventional emission spectrometer employs a dielectric pyrex tube having two electrodes extending therefrom to which an alternating electric field is applied at a high potential sufficient to cause an electric discharge. The gas sample is fed into the tube under continuous flow conditions and is excited by absorption of energy during the electric discharge. This results in the emission of radiant energy as the gas molecules drop from an elevated energy level to lower energy levels. The wavelengths of this emission are characteristic of the gas components excited by the absorption and release of energy. By filtering out unwanted wavelengths the intensity of the emission of any gas in the gas stream can be measured. In an argon gas stream, the concentration level of an impurity gas such as nitrogen can be measured by optically isolating light at the strongest characteristic wavelength for nitrogen i.e. at 337.1 nm and converting the separated optical signal to a corresponding electrical signal.
In conventional emission spectroscopy, the radiated output signal from the electric discharge source is modulated to produce an alternating signal using a mechanically rotating wheel sometimes colloquially referred to as a "chopper". The chopper, thus used to modulate the optical signal output from the electric discharge tube, produces a desired modulation frequency of e.g. 510 Hz. The modulated signal is then filtered to isolate the emission line 337.1 nm which is detected at the modulated frequency using signal electronics which includes a tuned amplifier to selectively amplify the 510 Hz modulated frequency signal and to reject other frequencies. A chopper has been used in emission spectroscopy for modulating the optical signal output of the silent electric discharge tube from its early inception. The function and need for a chopper in emission spectroscopy is described in detail in U.S. Pat. No. 3,032,654 issued May 1, 1962.
Another operation scheme for the operation of an emission spectrometer is described in detail in U.S. Pat. No. 5,412,467 issued May 2, 1995. In accordance with this disclosure, it was discovered that the use of a "chopper" and its function, which heretofore was deemed essential to the operation of an emission spectrometer, may be entirely eliminated. Instead the emissive radiation from the electrical discharge source was modulated by controlling the input frequency to the high voltage transformer and converted into a modulated electrical signal and amplified within a narrow frequency range centered at substantially twice the excitation frequency of the source of alternating voltage applied across the electric discharge source. This was reported to increase the sensitivity of detection of any gaseous impurity in the gas sample by an order of magnitude. In particular the range of detection of nitrogen in an argon gas sample using this concept was described as being extended to a level below 20 parts per billion (ppb).
The method pursuant to U.S. Pat. No. 5,412,467 for analyzing a continuously flowing gas stream at concentration levels extending below 20 parts per billion using emission spectroscopy comprises the steps of:
directing a sample of the gas stream through an electric discharge source; PA1 applying an alternating source of power across said electric discharge source at a preselected excitation frequency with said alternating power source having a peak voltage sufficient to sustain an electric discharge and to generate a wide radiation spectrum of emissive radiation from said gas stream; PA1 filtering said radiation spectrum to form an optical signal having a narrow radiation emission bandwidth corresponding to the stronger emission wavelength(s) of a preselected gas or vapor impurity to be analyzed; PA1 converting said optical signal into an electrical signal; PA1 selectively amplifying said electrical signal within a narrow frequency range centered at substantially twice said excitation frequency; and PA1 analyzing said selectively amplified electrical signal to determine the concentration level of the gas or vapor under analysis.
Utilizing this method, an improved gas emission spectrometer was disclosed comprising: a silent electric discharge source; means for feeding a gas sample through said discharge source at a preselected flow rate; power supply means for applying a source of alternating voltage across said silent discharge tube at a predetermined excitation frequency and of sufficient peak voltage predetermined excitation frequency and having sufficient peak voltage to sustain an electric discharge and generate emissive radiation from said gas stream over a wide radiation spectrum; means for optically filtering said radiation spectrum to form an optical signal having a narrow radiation emission bandwidth corresponding to the stronger emission wavelength(s) of a preselected gas impurity for detection in said gas sample; means for converting said optical signal into a corresponding electrical signal and analog amplifier means for selectively amplifying said electrical signal within a narrow frequency range centered at substantially twice the excitation frequency whereby the sensitivity of detection of said gas impurity is increased to a minimum detection level (MDL) of below 20 parts per billion (ppb).
Previous gaseous emission spectrometers, such as that described in U.S. Pat. No. 3,032,654, were designed to analyze a single impurity such as nitrogen in a sample gas such as argon. Accordingly, only a single analytical tube and a single photomultiplier detector was required. This analytical tube typically was rugged and relatively easy to assemble, as it consisted of thick plate glass and spacers epoxied together. The thickness of the plate glass and the spacers resulted in a relatively wide electrode gap of 9/16 inch. The electrode gap, however, is a key parameter in determining the voltage necessary to initiate the plasma discharge in the sample gas stream. At a gap of 9/16 inch, approximately 7000 VAC is necessary to sustain the plasma even in an easily excitable sample gas such as argon. In order to supply such a voltage continuously for long periods of time, a large, bulky transformer is required. In addition, in constructing such an analytical tube using conventional techniques, epoxy typically was used to seal joints and seams; with epoxy, off-gassing and risk of air leakage into the cell can degrade the sample gas purity.
Accordingly, there is a need to develop an improved analytical tube and method of analysis to detect multiple selected impurities in a gas stream of mixed gases under continuous flow conditions, particularly employing a configuration wherein the capability of multiple analysis does not result in the need for duplicative analysis equipment and the attendant increase in overall unit bulk and size.